For example FTX was able to claim, with a vague semblance of honesty, that it had assets to cover liabilities. But in fact what it did was transfer money to other companies that were part of the network, like Alameda, and get back tokens with a book value but no real market value, like Serum. On paper both companies had engaged in a fair transaction. In reality customer money got moved to another company that was then free to do with it what it wanted, all hidden behind the corporate structure.
Read https://archive.ph/TOgjK for Matt Levine explaining exactly how this actually worked.